In "cutting" a glass sheet by the conventional two-step score and break method, the geometry of the cutter wheel or scoring tool is such that a concentrated downward force is exerted along the path taken by the wheel as it is drawn across the sheet. This downward force depresses the glass surface slightly, inducing transverse tension forces in the glass at the surface. These tension forces alter tthe residual compressive stresses sub-adjacent the glass surface, and a fissure is produced along the line of the scoring tool. The sheet can then be conveniently broken out in the second step of this process by bending the sheet about an anvil or table edge so as to open up the fissure and separate the glass sheet material.
In carrying out the above-described conventional process, advantage is taken of the inherent stress distribution across the thickness of most glass sheet material. More particularly, and as a result of the fact that molten glass sheet material cools first at its outer surface, and in so cooling contracts, compressive stresses are created in the glass in the area beneath these exterior surfaces. Although these compressive stresses are reacted by internal tension stresses, and although most glass can be annealed to alleviate some such stresses, it has been found that when the glass thickness exceeds 1/2 inch the compressive stresses are still high enough to prevent creation of the necessary tension at the surface being scored, at least as a result of applying a scoring wheel of conventional geometry. Increasing the force applied to the cutter wheel will only tend to crush the glass. Additional time spent in the annealing process increases the cost of the glass material, and should not be necessary if the only reason for such additional annealing is to improve the glass cutting operation.
The general object of the present invention is to provide a economically feasible method and means for mechanically prestressing the glass to reduce the residual surface compression in the glass at the surface being scored, and thereby facilitate scoring of the glass by a conventional cutter wheel applied to the glass with conventional forces.